Spring Boot Interview Questions for Java Full Stack with React
25 Spring Boot questions with answers for java full stack with react roles — 6 easy, 14 medium, 5 hard.
About Spring Boot
Spring Boot is the most widely used Java framework for building REST APIs and microservices in India. This topic covers dependency injection, bean scopes, auto-configuration, profiles and configuration properties, Actuator, transactions, exception handling, testing with JUnit and Mockito, scheduling and Docker packaging, along with the key changes in Spring Boot 3 such as Jakarta packages and Java 17.
Sheet progressStart practicing
3 QUESTIONS FREESign up to unlock the full question bank and track your progress.Unlock every question and answer, then continue practising company-wise and package-wise.
✓ 3 questions free • Unlock the rest in seconds
01
What is Spring Boot and how is it different from the Spring Framework?
Spring Framework provides dependency injection, MVC, data access and transactions, but traditionally needed a lot of manual configuration and an external server. Spring Boot sits on top of Spring and adds auto-configuration, starter dependencies, an embedded Tomcat, Jetty or Undertow server, externalised configuration and production features like Actuator. You can run an application as a single executable jar with java -jar, which makes it ideal for microservices and containers.
02
Explain Inversion of Control and Dependency Injection.
Inversion of Control means the framework, not your code, creates and wires objects. Dependency Injection is how Spring does it: a class declares what it needs, usually through its constructor, and the container supplies the beans. For example, OrderService receives an OrderRepository instead of creating one with new. This reduces coupling, makes it easy to swap implementations and lets you pass mocks in unit tests.
It is a convenience annotation combining three annotations. @SpringBootConfiguration marks the class as a configuration source. @EnableAutoConfiguration tells Boot to configure beans based on the classpath and properties, for example a DataSource when a JDBC driver is present. @ComponentScan scans the package of the main class and its sub-packages for components. That is why the main class should sit in the root package of your project.
04
Create a scheduled job that runs every night at 2 AM in Spring Boot.
@SpringBootApplication
@EnableScheduling
public class App { }
@Component
public class CleanupJob {
private final SessionRepository repo;
public CleanupJob(SessionRepository repo) { this.repo = repo; }
@Scheduled(cron = "0 0 2 * * *", zone = "Asia/Kolkata")
public void removeExpiredSessions() {
repo.deleteByExpiresAtBefore(Instant.now());
}
}
// With several instances, use ShedLock so only one runs the job.
05
You need H2 for local development, MySQL for testing and PostgreSQL in production. How do you configure this?
Create application-dev.yml, application-test.yml and application-prod.yml, each with its own spring.datasource URL, username and driver. Keep shared settings in application.yml. Activate the right one with SPRING_PROFILES_ACTIVE in each environment. Read production credentials from environment variables, for example url: ${DB_URL}, instead of hard-coding them. For realistic integration tests, Testcontainers can start the same database engine used in production.
06
The application fails with NoSuchBeanDefinitionException for a service. How do you debug it?
Check that the class has @Service or @Component, or is created by a @Bean method. Make sure it is inside the package of the main @SpringBootApplication class or a scanned sub-package. If there are several implementations, use @Qualifier or @Primary. Also check @Profile or @Conditional annotations that may disable it in the current environment. Running with --debug prints the condition report that shows why a bean was not created.
singleton is the default: one instance per application context. prototype creates a new instance every time the bean is requested. Web applications also have request, session and application scopes, and websocket scope. Singleton beans must be stateless or thread-safe because many requests share them. If a singleton needs a prototype or request-scoped bean, inject an ObjectProvider or use a scoped proxy so each use gets the correct instance.
08
Why is constructor injection preferred over field injection?
Constructor injection makes dependencies explicit and lets fields be final, so the object is always fully initialised and immutable. It allows plain unit tests without Spring or reflection, because you can pass mocks to the constructor. A long constructor also signals that a class has too many responsibilities. Field injection with @Autowired hides dependencies and needs a container or reflection in tests. With a single constructor, @Autowired is not even needed.
09
How do profiles and application.yml work in Spring Boot?
Common settings go in application.yml, and environment-specific values go in files like application-dev.yml or application-prod.yml. You activate a profile with spring.profiles.active=prod, an environment variable SPRING_PROFILES_ACTIVE or a command-line argument. Profile files override the default values. Beans can also be limited with @Profile("dev"). Environment variables and command-line arguments have higher priority than files, which is useful in Docker and Kubernetes.
10
Mentoring: how would you help a junior developer who keeps pushing untested Spring code?
Situation: a junior on my team often broke the build with untested changes. Task: help him improve without demotivating him. Action: I paired with him for a few sessions to write JUnit and Mockito tests for his own features, shared a short checklist and set up a CI rule that blocks merges without passing tests. Result: within a month his pull requests passed review faster, and he later helped another new joiner.
Actuator adds production-ready endpoints for monitoring and management, such as /actuator/health, /actuator/info, /actuator/metrics and /actuator/prometheus. Kubernetes uses health liveness and readiness groups for probes. Metrics are collected through Micrometer and can be exported to Prometheus, Datadog or other systems. Only expose the endpoints you need with management.endpoints.web.exposure.include and secure them, because endpoints like env or heapdump can leak sensitive data.
12
What changed when moving from Spring Boot 2 to Spring Boot 3?
Spring Boot 3 requires Java 17 or later and is built on Spring Framework 6. The biggest migration task is the switch from javax.* to jakarta.* packages for persistence, validation and servlet APIs. Spring Security 6 removed WebSecurityConfigurerAdapter in favour of SecurityFilterChain beans. Boot 3 also supports GraalVM native images, Micrometer observation and tracing, and from 3.2 the RestClient and virtual thread support.
13
Write global exception handling for a REST API in Spring Boot.
Spring Boot 2.6 and later forbid circular references by default. The real fix is design: two services depending on each other usually means a responsibility is in the wrong place. Extract the shared logic into a third bean, or use an application event so one service notifies the other without a direct reference. Using @Lazy or allowing circular references in properties hides the problem and should only be a temporary workaround.
18
Database passwords and API keys are committed in application.properties. How do you fix this?
Remove the secrets from the repository, rotate the exposed credentials immediately, and clean the history if needed. Replace values with placeholders such as ${DB_PASSWORD} and supply them through environment variables, Kubernetes Secrets or a vault like HashiCorp Vault or a cloud secrets manager. Add a pre-commit secret scanner, keep local values in an ignored file, and give each environment its own credentials with least privilege.
19
Explain the architecture of your last full stack project end to end.
In my last project, a React single-page app called a Spring Boot REST API secured with JWT. The API followed controller, service and repository layers, used Spring Data JPA with MySQL and Flyway for migrations, and exposed Swagger documentation. I deployed it with Docker on a cloud VM behind Nginx. My contribution was the order module and payment status flow, and I also wrote the unit tests and CI pipeline.
20
Tell me about a time you delivered a feature under a tight deadline.
Situation: a client demo was moved forward by a week and the reporting module was not ready. Task: deliver the core report screens and APIs in five days. Action: I split the work into must-have and later items with my lead, reused existing query components, and wrote tests for the critical paths only. Result: the demo went well, and the remaining features shipped in the next sprint without bugs.
21
How does Spring Boot auto-configuration work internally?
Spring Boot reads candidate configuration classes listed in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports inside starter jars. Each class is guarded by conditions such as @ConditionalOnClass, @ConditionalOnMissingBean and @ConditionalOnProperty, so beans are created only when the right library is present and you have not defined your own. You can inspect the decisions with the --debug flag or the Actuator conditions endpoint and exclude classes with exclude attributes.
22
How does @Transactional work, and why does calling it from the same class not work?
Spring wraps the bean in a proxy. When an external caller invokes a @Transactional method, the proxy starts a transaction, calls the method and commits, or rolls back on a RuntimeException or Error. If a method in the same class calls another @Transactional method through this, the call skips the proxy, so the annotation is ignored. Fix it by moving the method to another bean or restructuring the boundary. The annotation also has no effect on private methods.
23
Write a controller slice test with @WebMvcTest and MockMvc.
Measure first with the startup Actuator endpoint or ApplicationStartup to see which beans are slow. Common causes are heavy component scanning, eager database or remote calls in @PostConstruct, Hibernate validating a large schema and unneeded auto-configurations. Narrow scan packages, defer work with lazy initialisation or ApplicationReadyEvent, exclude unused starters and use Class Data Sharing. For serverless workloads, consider GraalVM native images or CRaC.
25
Your @Transactional method threw an exception but the data was still saved. Why?
By default Spring rolls back only on unchecked exceptions and Errors, so a checked exception such as IOException commits the transaction. Use @Transactional(rollbackFor = Exception.class) or wrap it in a runtime exception. Other causes are catching the exception inside the method, calling the method from the same class so the proxy is bypassed, a non-public method, or a table engine that does not support transactions such as MyISAM.
Frequently asked questions
How important is Spring Boot in Java Full Stack with React interviews?
Spring Boot is the most widely used Java framework for building REST APIs and microservices in India. This topic covers dependency injection, bean scopes, auto-configuration, profiles and configuration properties, Actuator, transactions, exception handling, testing with JUnit and Mockito, scheduling and Docker packaging, along with the key changes in Spring Boot 3 such as Jakarta packages and Java 17.
How should I use the Spring Boot sheet?
Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.